The present invention relates generally to homopolar machines utilizing liquid metal for electrical contact between current collectors and a rotor, and, more particularly, to apparatus for confining liquid metal and maintaining electrical contact between the rotor and stator at all angular velocities, including zero.
In machines such as homopolar motors utilizing high current density field coils, high current-carrying capacity is required for making electrical connections between the rotor and the stator. It is well known in the art to utilize liquid metal current collectors for such machines due to the high current-density capacity of such collectors, for example, 3,000 to 10,000 amperes per square inch, by the liquid metal as compared to approximately one-tenth of this current density in the case of conventional solid carbon brushes. Further, solid brushes exhibit an order of magnitude higher voltage drop than that of liquid metal brushes, even at the lower current density. As a result of this higher voltage drop, solid brushes generate more heat than liquid metal brushes and are prone to fail. To provide the high current carrying capacity required by homopolar motors, a large number of solid brushes are necessary. The current is not necessarily shared equally be all of the brushes resulting in unequal heating and wear of the brushes. Moreover, a failure of one brush allowing debris to be transported around the slip ring may result in catastrophic failure of all the brushes.
In high current, high speed operation of homopolar machines, it has been common practice to provide a constant supply of liquid metal by pumping it to the stator and rotor current collector sites. This ensures that the current collectors are continuously wetted. At high speeds of operation, the centrifugal force tends to drive the liquid metal from between the current collector surfaces. Also, at high current densities, the pinch-effect tends to expel liquid metal from the collector surfaces. An additional problem experienced when utilizing liquid metal current collectors in homopolar machines is that the stray or non-radial components of the magnetic field from the stator can create Lorenz forces across the surface of the rotor collector in the axial direction, tending to eject the liquid metal from the collector site. If these forces cause the liquid metal to escape from the current collector sites, the current collectors would begin to run dry, thereby raising the current density to a point where arc-over and/or welding between the stator and rotor collector areas would occur. Such a result would severely damage the current carrying contact surfaces and would ruin the machine.
Liquid metal circulation means and methods are known in the prior art for circulating liquid metal around current collector sites and providing the necessary confinement of liquid metal in containment cages. In U.S. Pat. No. 4,241,273, Hatch teaches a homopolar machine having a plurality of compliant nonconductive cages affixed to the stator collector ring. Liquid metal is confined in the space defined axially and circumferentially by the cage walls and radially by the radially inner surface of the stator and the radially outermost surface of the rotor collector ring. The liquid metal is in contact with stator fins protruding radially-inwardly into each cage, respectively, and the radially-outermost surface of the rotor collector ring.
Both of the above-described prior art homopolar machines require a continuous supply of liquid metal to the enclosed area to replace liquid metal lost by leakage through the nonconductive walls or drawn out between the enclosure walls and the rotor outer surface by the rotation of the rotors in order to insure continuous contact over the entire current carrying area. One such method is disclosed in U.S. Pat. No. 4,207,486 issued June 10, 1980 to Burton D. Hatch and assigned to General Electric Company. Hatch teaches a disk-type homopolar machine which utilizes the pressure head created by rotation of the rotor disks in frictional contact with a liquid metal in radially-outer current collectors to pump liquid metal radially-inwardly toward radially-inner current collectors. Thus, a constant supply of liquid metal for the current collectors is provided by dynamic pumping through passageways through each stator disk extending from the region adjacent to a radially-outer collector to the region adjacent a radially-inner collector.
It is also known in the prior art to confine the liquid metal in clearly identified contact areas between the rotor and stator current collector rings by use of rigid or compliant nonconductive cages or containment walls disposed between the rotor and stator rings. U.S. Pat. No. 4,241,272 issued on Dec. 23, 1980 to Robert A. Marshall and assigned to General Electric Company discloses confinement of liquid metal by a rigid nonconductive enclosure. U.S. Pat. No. 4,241,273 issued on Dec. 23, 1980 to Burton D. Hatch and assigned to General Electric Company also discloses confinement of a liquid metal in continous contact with a defined surface.